1. Field of the Invention
This invention relates to computers and more particularly to feature boards for insertion into a slot of a computer to provide increased or enhanced function.
2. Description of the Related Art
It is well known to provide a computer, such as a PS/2 brand computer from International Business Machines Corp. with slots including one or more bus connectors to permit feature boards with associated circuitry to be connected to the computer bus to permit the user to expand the computer's capabilities. These slots provide physical support for the feature boards and the bus connector(s) is arranged to mate with a connector on the feature board when the board is fully inserted into the slot.
In certain computers, such as the IBM PS/2 Model 80, two different size bus connectors are provided in respective slots. These different connectors reflect differences in the number of parallel bits allocated to data and memory addressing. Usually the bits are allocated in multiples of eight and in the case of the Model 80 there are slots which provide for 16 parallel bits of data and addressing as well as slots which provide for 32 parallel bits. It is also known to provide a flag bit position to indicate when a command sent over the bus is a 16 or a 32 bit command (e.g. logic 1=32 bit and logic 0=16 bit) so that a board in a 16 bit slot may detect commands that are outside its range and take no action. This permits a 16 bit board to avoid a situation where, seeing only a portion of a command or address, it proceeds with an improper action.
This flag bit position does not however provide any protection in the event a 32-bit board is inserted in a 16-bit slot. In that case the board will proceed as if it is in a 32-bit slot and interpret binary signals at the missing positions on the bus connector improperly.
It is also known to steer data between two data busses of different size but such steering is typically effected by fixed interface logic recognizing that the bus definition on either side will remain the same.